This invention relates to a color error suppression apparatus and method for a color television camera in which color component signals representing first and second colors of a color image are reproduced by utilizing a difference of modulation phase relationships of the color component signals, one of which changes in successive scan lines.
An apparatus of the type employing a single pick up tube for producing a color video signal by the method of processing successive line signals produced by line scannings is shown, for example, in U.S. Pat. No. 3647943 or Japanese Published Pat. No. 45-8699. In those apparatuses a composite image is formed on a photoconductive surface, which has a first color such as blue spatially modulated in a first line pattern by one striped color filter, and a second color such as red spatially modulated in a second line pattern having a different angular relationship from the first line pattern by another striped filter. The signals representing the composite image are produced by scanning such an image. A modulated component of the signals is then processed to recover first and second color component signals in accordance with the first and second phase relationships in the successive line signals, respectively. The signals representing the luminance of the color image are produced from an unmodulated component of the composite signal through a low-pass filter and the third color signal such as a green signal, can be produced by operating upon the first and second color signals and the liminance signal.
In such an apparatus, when the video pattern of the object has relationships in the successive lines of scan such as in a vertical color bar pattern, the reproduction of each color may be achieved faithfully. On the contrary when the video pattern has no relationship or even partial relationship in the successive lines such as in horizontal color bar pattern including vertical edge portions therein, color errors take place at the edge portions in the reproduced image on a picture screen of a color television reproducing apparatus.
FIG. 1 is a block diagram of a color television camera which makes use of the Japanese Published Pat. No. 45-8699, wherein the pick up tube 1 comprises the striped color filters as shown schematically in FIG. 2. One of them, which is disposed perpendicular to the direction of horizontal line scanning contains a plurality of striped filter element pairs, W-elements which are transparent, and Ye-elements which have characteristics causing them to pass red and green light. The other striped color filter, which is disposed so as to have the elements in a different angular position from the element of the W - Ye striped filter relative to the direction of scanning, contains plurality of striped color filter element pairs, W - elements, and Cy - elements which have characteristics causing them to pass blue and green light. This W - Cy striped filter is placed over the W - Ye striped filter in such a position that the modulated red signals produced by the line scans have a phase relationship substantially 180.degree. out of phase in successive horizontal scan lines I, II, etc.
The output signal from the pick up tube 1 is amplified by a pre-amplifier 2 and supplied to a low-pass filter 3 to produce a luminance signal Y(t), and to another low-pass filter 4 to produce a luminance signal. Y.angle.(t) which is used to produce color difference signals, and is also supplied to a band-pass filter 5. The blue component signal B(t) is obtained from an adding circuit 8 including an amplifier and a demodulator by adding the output of the band-pass filter 5 and that of the 1 H-delay line 6. On the other hand, the red component signal R(t) is obtained from a subtractive circuit 7 including an amplifier and a demodulator by subtracting the output of the 1H-delay line 6 from that of the band-pass filter 5. In the encoder 9, the color difference signals B-Y and R-Y are produced from the color component signals R(t) and B(t) and the luminance signals Y .angle. (t). They are modulated by balanced modulators then mixed with the luminance signal Y(t) to generate the NTSC standard signal.
In this color television camera, the signal produced on the scan line I as shown in FIG. 2 is expressed by the following equation: ##EQU1## and the signal produced on the next scan line II is expressed as: ##EQU2## where, G, R and B represent green, red and blue components, respectively, m.sub.b .multidot.B and m.sub.r.multidot.R represent modulated blue and red components, and .omega.= 2.pi.f, f is the spatial frequency in cycles of modulation.
When an image pattern, which has the sequence black-white-black in the vertical direction and including two vertically edges as shown in FIG. 3 is scanned, the output signals Sa(t), Sb(t), Sc(t), Sd(t), and Se(t) from the pick up tube 1 are shown by the following equations in the scan lines a,b,c,d and e on the assumption that the scan lines correspond to the scan lines I, II, I, II, I in FIG. 2, respectively: ##EQU3##
In this case, the signals Y.angle.(t), R(t) and B(t) reproduced from the low-pass filter 4, the adding circuit 8 and the subtractive circuit 7 in FIG. 1 are shown in FIG. 4a FIG. 4b and FIG. 6, respectively for the condition that m.sub.r.multidot.R =m.sub.b .multidot.B. A color error that is magenta occurs at the edge portion where the image pattern changes from white to black.
On the other hand if each scan line a, b, c, d, e corresponds to the scan lines II, I, II, I, II in FIG. 2, respectively, the output signals of the pick up tube 1 are shown in following equations: ##EQU4##
In this case, the signals Y.angle.(t), R(t) and B(t) are shown in FIG. 4, FIG. 4d and FIG. 4e, respectively, for the condition m.sub.r.multidot.R = m.sub.b .multidot.B. A color error, that is green occurs at the edge portion where the image pattern changes from black to white.
The above analysis has been simplified to the case in which the horizontal line scanning is carried out on the scan lines I or II in FIG. 2 where the color error becomes extreme. However, the line scanning may also be carried out on other lines different from I and II, so the reproduced red signal R(t) can be represented as the average of R(t) in FIG. 4b and FIG. 4d as shown in FIG. 5b and the reproduced blue signal B(t) can be represented as the average of B(t) in FIG. 4(C) and FIG. 4(e) as shown in FIG. 5(c).
On the other hand, when the modulated components are not equal (m.sub.R .multidot.R m.sub.b .multidot.B) which can be expressed as m.sub.R .multidot.R =n.multidot.m.sub.b .multidot.B (the value of n depends on the characteristics of the pick up tube and the color filter, and on the color temperature of the image) the gain of the adding circuit 8 must be n times that of the subtractive circuit 7 so that the amplitude of the signals R(t) and B(t) will be equal at the input terminals of the encoder 9. Under these conditions the red and blue component signals R(t) and B(t) can be represented by the following equations, respectively, in the first aforementioned case. The signals produced by the scan on line b in FIG. 3 which are obtained from the equations (1) and (2): are EQU R(t) = m.sub. .multidot.B + m.sub. R .multidot. R = (1 + n) m.sub. B .multidot.B (11) EQU b(t) = n.multidot.(m.sub. B .multidot.B + m.sub. R .multidot.R) = .sub.n .multidot.(1 + n).multidot.m.sub. B .multidot.B (12)
the signals produced by the scan of the next line are c, which are obtained from the equations (2) and (3): EQU R(t) = 2.multidot.m.sub. R .multidot.R = 2.multidot.n.multidot.m .sub. .multidot.B (13) EQU b(t) = 2.multidot.n.multidot.m.sub. B .multidot.B (14)
the signals reproduced by the scan line e are same as the equations (11) and (12). If n = 3, the signals Y.angle.(t), R(t) and B(t) are shown in FIG. 6a, FIG. 6b and FIG. 6c, respecitively. Therefore, color errors which are bluish occur at both edge portions where the image pattern change from black to white and from white to black in this first case.
In the second aforementioned case when scan lines a, b, c, d, e in FIG. 3 corresponds to the scan lines II, I, II, I, II in FIG. 2, respectively, the red and blue component signals R(t) and B(t) can be represented by the following equations.
The signals produced by the scan 4 line b in FIG. 3 which are obtained from the equations (6) and (7) are: EQU R(t) = m .sub.r .multidot. R - m.sub.B .multidot. B = (n-1) m.sub.B .multidot.B (15) EQU b(t) = n.multidot.( m.sub.R .multidot.R - m.sub.b .multidot.B) = n.multidot.(n - 1) .multidot.m.sub.B .multidot.B (16)
the signals produced by the scan of the next scan line c which are obtained from the equations (7) and (8) are: EQU R(t) = 2 m.sub.R .multidot.R = 2.multidot.n.multidot.m.sub.B .multidot.B (17) EQU B(t) = 2.multidot.n.multidot.m.sub.B .multidot.B (18)
The signals produced by the scan 4 line e are the same as the equations (15) and (16). On condition n =3, the signals Y.angle. (t), R(t) and B(t) are shown in FIG. 7A, FIG. 7b and FIG. 7c, respectively The color errors also occur in the case at both edges.
As the line scanning may be carried out on the other lines in general, the color component signal R(t) obtained in practice may be represented as the average of R(t) in FIG. 6b and FIG. 7b as shown in FIG. 8b, and B(t) may be represented as the average of B(t) in FIG. 6(c) and FIG. 7(c) as shown in FIG. 8(c).
In any case, these color errors take place at the vertically spaced edge portions where the video pattern has no relationship in successive lines so long as the color error suppression method such as that of this invention is not in use.